1. Field of the Invention
The present invention relates to an electric furnace extension method for extending an optical fiber glass body (preform) so as to obtain a predetermined outer diameter, and to an apparatus of the same.
Note that, in the present specification, an xe2x80x9coptical fiber glass bodyxe2x80x9d includes, other than the usual glass body for extension, an extension optical fiber glass body obtained by extending this glass body, i.e., a xe2x80x9cpreformxe2x80x9d.
2. Description of the Related Art
Usually, an optical fiber is produced by synthesizing an optical fiber porous glass body by a VAD process or an external CVD process, then dehydrating and sintering the optical fiber porous glass body to obtain a transparent glass body for the optical fiber, extending this to obtain an outer diameter suited for wire-drawing to form a drawn optical glass body (referred to here as a xe2x80x9cpreformxe2x80x9d), and then wire-drawing this preform to form an optical fiber including a core and a cladding.
Heretofore, the optical fiber glass body has been drawn by heat extension of the optical fiber glass body using a burner using an oxyhydrogen flame as a heat source, that is, the burner extension method. In this burner extension method, control of the outer diameter of the extended optical fiber glass body is relatively easy, but the extension rate is slowxe2x80x94usually 8 to 10 mm/min.
Due to recent technical advances and along with demands for improvement of manufacturing efficiency, the outer diameter of the porous optical fiber glass body synthesized by the VAD process or external CVD process has become much greater than in the past. Along with this, it suffers from the disadvantage in terms of the heat in the burner extension method using a burner using an oxyhydrogen flame as the heat source in the case of an optical fiber glass body having an outer diameter before extension of more than a certain value.
For this reason, a heat extension method using a heating furnace having a larger heating capacity than a burner, more specifically an electric heater (hereinafter referred to as the xe2x80x9celectric furnace extension methodxe2x80x9d) is being adopted for optical fiber glass bodies having an outer diameter before extension of more than a certain value. Such an electric furnace extension method can increase the extension rate since the heating capacity is large. For example, in contrast to the extension rate of the burner extension method of 8 to 10 mm/min, the extension rate is 30 mm/min or more in the electric furnace extension method. For this reason, the electric furnace extension method is advantageous from the viewpoint of productivity not only for the extension of an optical fiber glass body having a large outer diameter, but also for an extension glass body having a small outer diameter.
Note that in this electric furnace extension method, since the heating range is wide, it is limited to use for vertical bodies long in the vertical direction. Accordingly, the preform is extended from top to bottom.
In this electric furnace extension method, it is necessary to connect an extension-glass-body support rod (pulling glass member) to the end portion of the optical fiber glass body or the extension glass body (preform) before extension. In the past, this connection was generally achieved in a separate step of flame extension, but it is preferable to achieve this connection in the same electric furnace extension process.
Where this connection is performed in the same electric furnace extension process, however, it suffers from the disadvantage that this connection is difficult. That is, it is necessary to align the axial center of the end portion of the glass body with the axial center of the end portion of the extension support rod (pulling use glass member), but the centers tend to be offset in connection. When the axial centers are offset in connection, the distribution of stress in the cross-section of the connection portion becomes nonuniform, so trouble such as a bending in the drawn body or breakage of the connection portion sometimes occurs.
The axial alignment at the time of extension an optical fiber glass body using an electric furnace is usually performed as follows.
The two glass members (extension use glass body and pulling use glass member), one end of each of which is held elsewhere, are inserted into the heated furnace from reverse directions to each other and made to abut against each other. The abutting ends of the two are heated and fused to each other, then the gripping member gripping the pulling use glass member is moved downward at a predetermined speed while moving the gripped portion of the extension use glass body downward at a constant speed so as to stretch the extension use glass body and extend it to the predetermined outer diameter.
FIG. 1 is a view of an example of the configuration of this type of extension apparatus for an optical fiber glass body of the related art.
In FIG. 1, 91a denotes an extension use glass body (or optical fiber glass body) to be extended, and 91b denotes a dummy pulling use glass member (extension-out use glass member). Reference numeral 93 denotes a furnace body. A furnace pipe 94 containing an electric heater is disposed in the furnace body 93.
One end portion of each of the extension use glass body 91a and the pulling use glass member 91b is gripped and affixed by gripping members 95 and 96 comprising for example three-claw chucks. The gripping members 95 and 96 can be moved by a moving stand 97 to which the gripping members 95 and 96 are affixed in the vertical direction in a state illustrated in FIG. 1. Reference numeral 98 denotes a guide rail for guiding the moving stand 97.
The optical fiber glass body is extended (stretched) by the above optical fiber body extension apparatus as follows:
(1) The fixed ends of the extension use glass body 91a and the pulling use glass member 91b are gripped and affixed by the gripping members 95 and 96, respectively. The front ends of the free end portions of the affixed extension use glass body 91a and pulling use glass member 91b are axially aligned, then the two are moved into the furnace pipe 94 and made to abut against each other.
(2) The furnace pipe 94 is heated to the predetermined temperature to heat and fuse the abutted end surfaces of the two to join them, then the gripping member 95 gripping the pulling use glass member 91b is moved downward to extend the extension use glass body 91a to the predetermined outer diameter.
(3) The axial alignment of the extension use glass body 91a and the pulling use glass member 91b is carried out when heating and fusing the abutted end surfaces of the two in the furnace pipe 94 by the extension apparatus. For this axial alignment (centering), the fixed end portions of the extension use glass body 91a and the pulling use glass member 91b are for example moved delicately by hand within the range of free play of the gripping members 95 and 96, for example, the three-claw chucks, to center the axial centers of the front ends of the free end portions, then are fixed in place. The fixed front ends of the free end portions are then made to abut in the furnace pipe 94. The state of axial alignment is confirmed visually from an observation window 94A provided in the furnace pipe 94.
This centering method of the related art takes a long time. Also, also the precision of alignment is not very good. For example, an axial deviation of xc2x11 to xc2x12 mm (5 to 10% of the diameter of the glass member) occurs in the horizontal plane. If the precision of the axial alignment is poor, the distribution of stress at the cross-section of the connection portion of the extension use glass body 91a and the pulling use glass member 91b will become nonuniform at the time of extending, and trouble such as occurrence of bending in the extended fiber glass body or breakage of the connection portion may occur.
Further, where the optical fiber glass body is extended by the electric furnace extension method explained above, bending or waviness sometimes occur in the extended fiber glass body (preform) or variation sometimes occurs in the outer diameter, so the outer diameter sometimes deviates from the prescribed value.
Further, improvement of the extension rate of preforms has also been demanded from the viewpoint of productivity.
An object of the present invention is to overcome the disadvantages encountered when an optical fiber glass body and a pulling use glass member are connected in the electric furnace extension process and provide an electric furnace extension method and extending apparatus for an optical fiber glass body capable of easily and simply performing axial alignment (centering) of the front ends of the free end portions of the glass body and pulling use glass member to be abutted and connected in an electric furnace. More specifically, the object thereof is to provide an electric furnace extension method for fixing the optical fiber glass body to the extending apparatus, connecting the two in the electric furnace, and then starting the extension.
Another object of the present invention is to provide an electric furnace extension method and extending apparatus for an optical fiber glass body capable of improving the precision of the extended outer diameter of the optical fiber glass body.
Still another object of the present invention is to provide an electric furnace extension method and extending apparatus capable of increasing the extension rate of an optical fiber preform.
According to a first aspect of the present invention, there is provided an apparatus for extending an optical fiber glass body, comprising: an electric furnace for heating an extension optical fiber glass body to be extended; a first holding member positioned above said electric furnace, and holding a fix end of a upper portion of said extension optical fiber glass body to be extended; a second holding member positioned below said electric furnace and holding a fix end of a lower portion of a pulling glass member; a first axial center alignment mechanism provided between said electric furnace and said first holding member and for aligning an axial center of a tip of a free end of a lower portion of said extension optical fiber glass body; and a second axial center alignment mechanism provided between said electric furnace and said second holding member and for aligning an axial center of a tip of a free end of a upper portion of said pulling glass member, an axial center of said axial center alignment mechanism being met with an axial center of said second axial center alignment mechanism.
The first axial center alignment mechanism may comprise a pair of axial center alignment holders provided a pair of V-shaped grooves facing to a longitudinal direction of said extension optical fiber glass body at facing centers thereof, and having self axial center alignment function, a pair of supporting bars provided at facing portions to support said pair of axial center alignment holders, and a pair of bases having movement mechanisms for moving said pair of axial center alignment holders and said pair of supporting bars as a unit in a horizonal direction.
The second axial center alignment mechanism may also comprise a pair of axial center alignment holders provided a pair of V-shaped grooves facing to a longitudinal direction of said pulling glass member at facing centers thereof, and having self axial center alignment function, a pair of supporting bars provided at facing portions to support said pair of axial center alignment holders, and a pair of bases having movement mechanisms for moving said pair of axial center alignment holders and said pair of supporting bars as a unit in a horizontal direction.
According to a second aspect of the present invention, there is provided a process of connecting and fixing an extension optical fiber glass body to be extended and a pull glass member in an apparatus for extending the optical fiber glass body, comprising: an electric furnace for heating the optical fiber glass body; a first holding member positioned above said electric furnace, and holding a fix end of a upper portion of said extension optical fiber glass body to be extended; a second holding member positioned below said electric furnace and holding a fix end of a lower portion of a pulling glass member; a first axial center alignment mechanism provided between said electric furnace and said first holding member and for aligning an axial center of a tip of a free end of a lower portion of said extension optical fiber glass body; and a second axial center alignment mechanism provided between said electric furnace and said second holding member and for aligning an axial center of a tip of a free end of a upper portion of said pulling glass member, an axial center of said axial center alignment mechanism being met with an axial center of said second axial center alignment mechanism, said process including the steps of:
provisionally fixing a fix end of a upper portion of said extension optical fiber glass body to said first holding member;
holding a tip of a free end of a lower portion of said extension optical fiber glass body by said first axial center alignment mechanism and aligning an axial center of said tip of said free end;
holding and fixing said fix end of the lower portion of said extension optical fiber glass body by said first holding member;
releasing said pair of axial center holding members of said first axial center alignment mechanism and positioning the tip of the free end of said extension optical fiber glass body at a predetermined position in said electric furnace;
holding and fixing the fix end of the lower portion of said pulling glass member by said second holding member;
holding a tip of a free end of a upper portion of said pulling glass member by said pair of second axial center alignment mechanism having a self axial center alignment function to align the axial center of the tip of said free ends;
holding and fixing the fix end of the lower portion of said pulling glass member by said second holding member; and
releasing said pair of axial center holding members of said second axial center alignment mechanism and positioning the tip of the free end of said pulling glass member, to thereby coincide the tip of the free end of said extension optical fiber glass body and the tip of the free end of said pulling glass member to meet each axial center.
The process may include the steps of:
holding and fixing the fix end of the lower portion of said pulling glass member by said second holding member;
holding a tip of a free end of a upper portion of said pulling glass member by said pair of second axial center alignment mechanism having a self axial center alignment function to align the axial center of the tip of said free ends;
holding and fixing the fix end of the lower portion of said pulling glass member by said second holding member;
releasing said pair of axial center holding members of said second axial center alignment mechanism and positioning the tip of the free end of said pulling glass member,
provisionally fixing a fix end of a upper portion of said extension optical fiber glass body to said first holding member;
holding a tip of a free end of a lower portion of said extension optical fiber glass body by said first axial center alignment mechanism and aligning an axial center of said tip of said free end;
holding and fixing said fix end of the lower portion of said pulling glass member body by said first holding member;
releasing said pair of axial center holding members of said first axial center alignment mechanism and positioning the tip of the free end of said extension optical fiber glass body at a predetermined position in said electric furnace; to thereby coincide the tip of the free end of said extension optical fiber glass body and the tip of the free end of said pulling glass member to meet each axial center.
The process may also include the steps of:
provisionally fixing a fix end of a upper portion of said extension optical fiber glass body to said first holding member, and holding and fixing the fix end of the lower portion of said pulling glass member by said first holding member;
holding a tip of a free end of a lower portion of said extension optical fiber glass body by said first axial center alignment mechanism and aligning an axial center of said tip of said free end; and holding a tip of a free end of a upper portion of said pulling glass member by said pair of second axial center alignment mechanism having a self axial center alignment function to align the axial center of the tip of said free ends;
holding and fixing said fix end of the lower portion of said extension optical fiber glass body by said first holding member; and
holding and fixing the fix end of the lower portion of said pulling glass member by said second holding member; and
releasing said pair of axial center holding members of said first axial center alignment mechanism and positioning the tip of the free end of said extension optical fiber glass body at a predetermined position in said electric furnace; and
releasing said pair of axial center holding members of said second axial center alignment mechanism and positioning the tip of the free end of said pulling glass member, to thereby coincide the tip of the free end of said extension optical fiber glass body and the tip of the free end of said pulling glass member to meet each axial center.
According to the present invention, there is also provided an apparatus for extending an optical fiber body, comprising: a upper holder for holding a upper end of an extension optical fiber glass body to be extended; a lower holder, provided at a position coinciding an axial center with that of said upper holder and facing to said upper holder, holding a lower end of a pull glass member connected to a lower end of the extension optical fiber glass body, an axial center of said pull glass member being coincided an axial center of said extension optical fiber glass body, and said lower holder being pulled toward a lower portion, in response to an extension of said extension optical fiber glass preform; an electric furnace, provided between said upper holder and said lower holder, heating said extension optical glass preform; a moving means moving at least said lower holder toward a low position to pull said extending optical fiber glass preform; and a control means. The control means controls the following, when extending said optical fiber glass preform in said electric furnace, and after junction of the lower end of said optical fiber glass body and the upper end of said pull glass member by heat-melting manner,
moving the maximum temperature portion of said electric furnace to the junction portion to heat and connect the junction portion, and
controlling said movement means to lower said lower holder, to thereby move the maximum temperature portion of said electric furnace from said junction portion to the extending portion of said optical fiber glass preform.
Preferably, a dummy member is connected to the lower end of said extension optical fiber glass body, a diameter of a lower tip of said dummy member being smaller than a diameter of the upper tip of said pulling glass member, a diameter of a upper tip of said dummy member being substantially equal to or close to a diameter of said extension optical fiber glass body, said dummy member has a semi-conical shape in the upper tip and said dummy member being formed by a material equal to that of said extension optical fiber glass body, and said control means controls the movement of said moving means to move said lower holder downward to thereby extend said extension optical fiber glass body connected said dummy member at the lower tip thereof.
Preferably, the diameter of said lower tip of said dummy member is approximately xc2xd to ⅓ of the diameter of said upper end of said pulling glass member.
According to the present invention, there is further provided an apparatus for extending an extension optical fiber glass body, comprising: a upper holder for holding a upper end of an extension optical fiber glass body to be extended; a lower holder, provided at a position where an axial center thereof coincides with and an axial center of said upper holder, holding a lower end of a pulling glass member a upper end of which is connected to a lower end of said extension optical fiber glass body to coincide both axial centers, and movable downward in response to the extension of said extension optical fiber glass body; an electric furnace provided between said upper holder and said lower holder and heating said extension optical fiber glass body; a temperature measurement unit for measuring the temperature of an extending portion of said extension optical fiber glass body; a speed meter for measuring an extension speed of said extension optical fiber glass body after extension; a moving means for moving at least said lower holder downward to pull said extension optical fiber glass body after extension; and a control means; the control means reads the temperature from the temperature measurement unit and the extension speed from said speed meter, and controls said moving means to continuously raise set extension speed to a predetermined steady extension speed from the beginning of the extension to the steady extension state.
Preferably, said control means controls the heat power of said electric furnace to raise the temperature of said electric furnace in response to the increase of said extension speed.
Preferably, the apparatus for extending an extension optical fiber glass body further comprises an extension shape measurement means for measuring a shape of said extending portion of said extension optical fiber glass body, and said control means reads the shape information from said extension shape measurement means and controls the movement speed of said moving means to control said extension speed and the heat power of said electric furnace to control said heat temperature, to thereby maintain a meniscus angle of said extending portion, said meniscus angle being determined by said read shape information.
Preferably, said control means controls said moving means and said electric furnace so that said extending meniscus angle is kept 2 to 4 degree.
According to the present invention, there is provided a method of extending an optical an fiber glass body in an electric furnace, the method including the step of raising an extension speed of an extended optical fiber glass body to a predetermined extension speed, from the beginning of the extension to the steady extension state.
The method may include a step of controlling said extension speed so that a meniscus angle at an extending portion of said optical fiber glass body is kept 2 to 4 degree.
The method may further include a step of raising a heating temperature of the electric furnace in response to the raise of said extension speed.